RE: [PATCH] static_call,x86: Robustify trampoline patching

David Laight
2021-11-03T10:01:44Z

From: Peter Zijlstra
> Sent: 03 November 2021 08:36
>
> On Tue, Nov 02, 2021 at 05:20:05PM -0700, Andy Lutomirski wrote:
> > I think that's a big mistake -- any sane ENDBR-using scheme would
> > really prefer that ENDBR to be right next to the actual function body,
> > and really any scheme would benefit due to better cache locality.
>
> Agreed, IBT/BTI want the landing pad in front of the actual function.
>
> > But, more importantly, IMO any sane ENDBR-using scheme wants to
> > generate the indirect stub as part of code gen for the actual
> > function.
>
> Sorta, I really want to be able to not have a landing pad for functions
> whose address is never taken. At that point it doesn't matter if it gets
> generated along with the function and then stripped/poisoned later, or
> generated later.
>
> As such, the landing pad should not be part of the function proper,
> direct calls should never observe it.
>
> Less landing pads is more better.

One problem is when a direct call is 'too far' for a call instruction.
IIRC this can happen in arm64 with modules (all 64bit except x86?).
So an indirect call has to be used instead - which needs the landing pad.
Although it may actually be better to put a trampoline (landing pad
+ near jump) elsewhere and have the module loader do the correct fixup.
(Is the loader already generating a trampoline in the module code?)
The function body can then be cache-line aligned - with its benefits.

Can't anything that can write instructions always use a retpoline
to implement a jump indirect to an arbitrary address?
(Not to mention just generating the required code rather than a call.)

AFAICT CFI is all about detecting invalid values in function pointer tables.
It doesn't really protect in any way from JIT code doing incorrect things.

David

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